prepare new stable release - updating the workflow

[ReimarBauer]

We're updating our development workflow with release 11.0.0 (see #3000). We will be using a single development branch, and the stable branch will only receive critical bug fixes, and security patches – ideally with corresponding updates to the develop branch. There will be no more merge requests from stable to development, nor any from development to stable.

We create major releases using a stable Git branch, and we'll either rename or remove the existing stable branch when doing so.

For the next major release 11.0.0

I need to

• Update version numbers
• pixi lock
• Update the CHANGES file
• Update our development documentation

Once I've renamed/recreated the stable branch, I document how it worked out so we can include that information in the next release, 11.0.1 documentation.